Border security must be a key part of any immigration reform package, Homeland Security Secretary Janet Napolitano said Tuesday.

However, Napolitano said other parts of an immigration bill, such as a path to citizenship for undocumented immigrants, should not be contingent on achieving certain metrics for border security, what some politicians have called a "trigger."

"I think that once people really look at the whole system and how it works, relying on one thing as a so-called trigger is not the way to go," Napolitano said at a breakfast hosted by The Christian Science Monitor. "There needs to be certainty in the bill so that people know when they can legalize and then when the pathway to citizenship, earned citizenship, would open up."

Border security remains a contentious issue between Democrats and Republicans in the immigration reform debate. A bipartisan Senate group crafting a comprehensive bill has made a path to citizenship contingent upon achieving border security metrics in order to satisfy Republicans who believe that the U.S.-Mexico border remains vulnerable to illegal immigration.

Napolitano's comments could inflame Republican critics, who argue that the Obama administration has not done enough to secure the border. But Napolitano contends that the border is more secure now than during any time in history.

At the same time, by the secretary's own admission, one of the most oft-cited statistics to measure border security is flawed.

The Department of Homeland Security (DHS) references the falling number of apprehensions along the border in recent years as evidence that fewer people are crossing illegally into the U.S. But the measure is imperfect, according to Napolitano.

The number of apprehensions can vary based on the amount of resources devoted to the border and the effectiveness of border operations. The DHS has its own method of measuring border security, Napolitano said on Tuesday.

"What we look for is a combination of the manpower, the technology we have, the aerial coverage, and infrastructure," she said.

However, that measure doesn't translate into a publicly available statistic.

Prior to 2010, the DHS gauged the success of its work along the border by counting the number of miles that were under what it called "operational security." But the department stopped using the measure two years ago, since Napolitano believed it didn't account for the vast buildup of agents and infrastructure on the southern border in recent years.

In place of the previous measure, Homeland Security planned to develop a new border security metric. But that project, known as the Border Condition Index, is behind schedule. A DHS official recently testified before a congressional committee that the index was unlikely to be completed in coming months.

"The Border Condition Index was a project we undertook because we thought just measuring apprehensions at the border was not in and of itself a total measure of what life was like at the border," Napolitano said on Tuesday. "You needed to look at things like property values and crime rates, and things of that sort, for the seven million or so people, particularly who live along the U.S.-Mexico border."

"That is, as it turns out, a very difficult thing to do in any kind of statistically significant way," Napolitano said.

The lack of a sound statistical measure for border security could muddy the debate over whether an immigration bill should contain a security threshold that must be reached before other parts of the bill can be enacted.

Apprehensions show how many people are being picked up by Border Patrol agents. But the figure is so relative to staffing and strategy that it is essentially meaningless without greater context.

"A real measure is more qualitative," Napolitano said. "Really, when you step back and think about the border, what you want is the ability to spot illegal traffic, particularly in the highly trafficked areas -- some parts of the border are virtually not trafficked at all -- and then the ability to respond.

"Using that measure of the plans we have, we're confident that the border is as secure as it's ever been," she continued. "But there is no one number that captures that. That is the problem, if you're looking for just one number."